Breakdown of old Delta - ok
Mentioning of dropped changes - ok
On the remaining delta mostly ack, I wonder as well about the cross-compile changes but keeping for now should be ok.

One thing that made me wonder is the systemd diff for #1488962 still carried.
First of all we should try to still carry the LP reference just without the # to not trigger bug updates. This is not too important as "way down in the CL" I could find it.
But we have a native systemd service for a while now, so this is not more needed.
I dropped the file and systemctl daemon-reload
Then I find the conf no more relevant as expected, still everything reported in the old bug is working just fine.
In particular the native service is type=forking and RemainAfterExit defaults to no
That said, please drop that part of the Delta

One more thing - Is the feature that brotli would provide great enough to let us want to open a MIR for it to one day follow Debian on it?

I did my own build and tests and all seemed good to me.
Once you have adapted the Delta I'd ask you to check if test-apache2.py of [1] still works fine.
If that is good as well, then we can go on with the merge.

[1]: git+ssh://git.launchpad.net/qa-regression-testing
Other than these I'm happy with it, so it should be good after cleaning up the above.